1. The Field of the Invention
The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention is directed to a method of forming polysilicon having a crystal structure with a desired surface roughness useful in the manufacture of semiconductor memory devices.
2. The Relevant Technology
Polvsilicon is used in a variety of ways in the design and fabrication of integrated circuits. Polysilicon is used in forming gates, contacts, capacitors, and many other circuit structures.
The capacitance of polysilicon is influenced significantly by its structure. The surface roughness of polysilicon must be consistently controlled to achieve desired capacitance. Capacitance of capacitors constructed with polysilicon generally increases with surface roughness because of increased surface area.
Consistency in achieving desired polysilicon surface roughness in the production fabrication environment has proved somewhat difficult.
In accordance with the method of the present invention, polysilicon of a desired surface roughness is reliably and repeatably formed by providing a layer of doped or undoped amorphous silicon on a substrate and heating the substrate while monitoring the emission of the substrate relative to an expected emission attributable to the heating regime employed.
In conversion of a layer of amorphous silicon to polysilicon, the layer of amorphous silicon is heated in a chamber while the electromagnetic emission therefrom is monitored with a pyrometer. A signature change in the emission rate indicates formation of polysilicon. The silicon layer is heated using a heating element and control system to maintain a constant silicon laver temperature. The power delivered to the heating element automatically adjusts such that the constant temperature is maintained.
An increase in the monitored emission not attributable to the heating regime signals the transition from amorphous silicon to rough polysilicon. A decrease of the monitored emission that is not attributable to the heating regime signals a transition to smooth polysilicon. The increase and decrease in the monitored emission can be used to end the heating regime at the time at which the desired surface roughness of polysilicon is formed, or merely to passively monitor the process. The in situ monitoring of the present invention provides the advantages of greater control, reliability, and repeatability in forming polysilicon having a desired surface roughness and electrical and other characteristics related thereto.
The power delivered to the heating elements drops upon formation of polysilicon, which may be explained by an increase in the absorption of the silicon during its conversion from an amorphous to a crystalline form. As such, the formation of polysilicon can be detected by monitoring emission rates as well as by monitoring the power delivered to the heating elements.
It is prefprred to deposit silicon with a selected degree of crystallinity in order to better control the grain size of the resulting HSG polysilicon. If the silicon being deposited is completely amorphous silicon, grains of silicon that are larger than what may be desirable tend to form.
In certain applications, it is preferable to practice the invention with a silicon/germanium alloy instead of using pure silicon. A Si/Ge alloy is preferred for use in a capacitor storage node that is adjacent to a capacitor dielectric plate having a relatively high dielectric constant. The silicon/germanium alloy is highly conductive, reduces leakage through the dielectric plate, and would be particularly useful in DRAM cells in which leakage control is critical.
These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.